1. Field of the Invention
The present invention relates to node units and communications methods using the node units which link a trunk local area network (herein after "trunk LAN") to a number of branch local area networks (hereinafter "branch LANs") each having a number of terminal devices.
2. Description of the Prior Art
FIG. 6 shows a network system employing four conventional node units, wherein a trunk LAN is connected to four branch LANs for extending the communications area but minimizing an increase in the system traffic by a bridge function or transmitting only the frame necessary for the communications. The network system includes a trunk LAN 1; four branch LANs 2-1, 2-2, 2-3, and 2-4 each connected to the trunk LAN 1; five terminal devices T connected to the branch LANs, two for the branch LAN 2-1 and one for each of the branch LANs 2-2, 2-3 and 2-4, each terminal device having an address TA1, TA2, TA3, TA4, or TA5; four node units N1, N2, N3, and N4 for connecting the branch LANs 2-1, 2-2, 2-3, and 2-4 to the trunk LAN 1, each node unit having a filtering table TB1, TB2, TB3, or TB4 for storing terminal addresses of the terminal devices T connected to its own branch LAN.
Arrows 10, 11, 12, and 13 represent communications from the terminal device T of the terminal address TA1 to the terminal device T of the terminal address TA2; from the terminal device T of the terminal address TA1 to the terminal device T of the terminal address TA3; from the terminal device T of the terminal address TA1 to the terminal device T of the terminal address TA4; from the terminal device T of the terminal address TA1 to the terminal device T of the terminal address TA5, respectively.
FIG. 7 shows the format of a communications frame between terminal devices. The format includes a starting delimiter SD for indicating the start of a frame; a frame control FC for identifying the frame type; a destination address DA for identifying the address of a destination terminal; a source address (an address of a sender) SA for identifying the address of a sending terminal; information I; a frame error checking code CRC; and an ending delimiter ED for indicating the end of a frame.
Only the five terminal devices are used in the simple system of FIG. 6 but, in general, a large number of terminal devices are connected to each branch LAN, and a large number of node units are used. There are some differences between the branch LAN and the trunk LAN in the contents of a frame format in FIG. 7, such as code systems of the starting delimiter SD and the ending delimiter ED and definitions of the frame control FC, because of different communications protocols, but the basic contents, such as those of the destination address DA, the source address SA, and the information I, are identical.
In operation, the node units N1-N4 monitor the contents of all frames sent out on the branch LANs 2-1 through 2-4 and automatically form the filtering tables TB1-TB4. In other words, each node unit N1-N4 is able to identify the terminal devices connected to its own branch LAN by investigating the source address SA of a received frame format. In FIG. 6, the terminal addresses TA1 and TA2, TA3, TA4, and TA5 are automatically registered in the filtering tables TB1, TB2, TB3, and TB4, respectively. Then, each node unit N1-N4 receives all frames running on the branch LANs 2-1 through 2-4 and the trunk LAN 1 and determines whether the frame is to be transferred or not based on the contents of its filtering table TB1-TB4.
FIG. 8 shows a filtering process or how a node unit processes a frame received from a branch LAN. When a certain node unit Ni received a frame from a branch LAN 2-i, the node unit Ni retrieves the contents of a destination address DA in the frame from the filtering table TBi (Step 31). If the identical address has been registered in the filtering table TBi, the frame is erased because this frame is addressed to its own branch LAN (Step 32). If no identical addresses are registered, on the other hand, the frame is transferred to the trunk LAN 1 because it is addressed to another branch line (Step 33).
FIG. 9 shows another filtering process or how a node unit processes a reception from the trunk LAN. Upon reception of a frame from the trunk LAN 1, the node unit retrieves the contents of a destination address DA from the filtering table TBi in the frame in the same way as described above (Step 41). If the identical address has been registered in the table TBi, the node unit transfers the frame because it is addressed to its branch line from another (Step 42). If no identical addresses are registered, on the other hand, the node unit erases the frame because it is not addressed to its branch line (Step 43).
For example, the destination address DA of a frame indicated by the arrow 10 in FIG. 6 is the terminal address TA2. Accordingly, the frame is not transferred to another node by the above filtering process so that the frame is erased in the node unit N1. For example, the destination address DA of a frame indicated by the arrow 11 is the terminal address TA3. Consequently, the frame is transferred in the node units N1 and N2 by the filtering process but erased in the node units N3 and N4. The frames indicated by the arrows 12 and 13 are processed in the same way as described above.
In this way, the node units make a proper decision whether a frame is to be transferred or erased for communications among the respective terminal devices so that only the necessary frame runs on the trunk and branch LANs, thus minimizing an increase in the traffic.
In the above conventional communications method it is possible to extend the communications area by connecting branch LANs to the trunk LAN, but it has been difficult to maintain security of information because the terminal device of the specified node is always ready to respond. Also, it has been impossible to inhibit communications between specified branch LANs at will. Conversely, it has been impossible to release the inhibition for only the specified terminal devices to communicate between the inhibited branch LANs.